Purine uptake and utilization by the avian malaria parasite Plasmodium lophurae

Crystal structure of truncated aspartate transcarbamoylase from Plasmodium falciparum

The de novo pyrimidine-biosynthesis pathway of Plasmodium falciparum is a promising target for antimalarial drug discovery. The parasite requires a supply of purines and pyrimidines for growth and proliferation and is unable to take up pyrimidines from the host. Direct (or indirect) inhibition of de novo pyrimidine biosynthesis via dihydroorotate dehydrogenase (PfDHODH), the fourth enzyme of the pathway, has already been shown to be lethal to the parasite. In the second step of the plasmodial pyrimidine-synthesis pathway, aspartate and carbamoyl phosphate are condensed to N-carbamoyl-L-aspartate and inorganic phosphate by aspartate transcarbamoylase (PfATC). In this paper, the 2.5 Å resolution crystal structure of PfATC is reported. The space group of the PfATC crystals was determined to be monoclinic P21, with unit-cell parameters a = 87.0, b = 103.8, c = 87.1 Å, α = 90.0, β = 117.7, γ = 90.0°. The presented PfATC model shares a high degree of homology with the catalytic domain of Escherichia coli ATC. There is as yet no evidence of the existence of a regulatory domain in PfATC. Similarly to E. coli ATC, PfATC was modelled as a homotrimer in which each of the three active sites is formed at the oligomeric interface. Each active site comprises residues from two adjacent subunits in the trimer with a high degree of evolutional conservation. Here, the activity loss owing to mutagenesis of the key active-site residues is also described.

Purine import into malaria parasites as a target for antimalarial drug development

Infection with Plasmodium species parasites causes malaria. Plasmodium parasites are purine auxotrophs. In all life cycle stages, they require purines for RNA and DNA synthesis and other cellular metabolic processes. Purines are imported from the host erythrocyte by equilibrative nucleoside transporters (ENTs). They are processed via purine salvage pathway enzymes to form the required purine nucleotides. The Plasmodium falciparum genome encodes four putative ENTs (PfENT1-4). Genetic, biochemical, and physiologic evidence suggest that PfENT1 is the primary purine transporter supplying the purine salvage pathway. Protein mass spectrometry shows that PfENT1 is expressed in all parasite stages. PfENT1 knockout parasites are not viable in culture at purine concentrations found in human blood (<10 μM). Thus, PfENT1 is a potential target for novel antimalarial drugs, but no PfENT1 inhibitors have been identified to test the hypothesis. Identifying inhibitors of PfENT1 is an essential step to validate PfENT1 as a potential antimalarial drug target.

A comprehensive model of purine uptake by the malaria parasite Plasmodium falciparum: identification of four purine transport activities in intraerythrocytic parasites

Plasmodium falciparum is incapable of de novo purine biosynthesis, and is absolutely dependent on transporters to salvage purines from the environment. Only one low-affinity adenosine transporter has been characterized to date. In the present study we report a comprehensive study of purine nucleobase and nucleoside transport by intraerythrocytic P. falciparum parasites. Isolated trophozoites expressed (i) a high-affinity hypoxanthine transporter with a secondary capacity for purine nucleosides, (ii) a separate high-affinity transporter for adenine, (iii) a low-affinity adenosine transporter, and (iv) a low-affinity/high-capacity adenine carrier. Hypoxanthine was taken up with 12-fold higher efficiency than adenosine. Using a parasite clone with a disrupted PfNT1 (P. falciparum nucleoside transporter 1) gene we found that the high-affinity hypoxanthine/nucleoside transport activity was completely abolished, whereas the low-affinity adenosine transport activity was unchanged. Adenine transport was increased, presumably to partly compensate for the loss of the high-affinity hypoxanthine transporter. We thus propose a model for purine salvage in P. falciparum, based on the highly efficient uptake of hypoxanthine by PfNT1 and a high capacity for purine nucleoside uptake by a lower affinity carrier.

Purine and pyrimidine transport in pathogenic protozoa: From biology to therapy

Purine salvage is an essential function for all obligate parasitic protozoa studied to date and most are also capable of efficient uptake of preformed pyrimidines. Much progress has been made in the identification and characterisation of protozoan purine and pyrimidine transporters. While the genes encoding protozoan or metazoan pyrimidine transporters have yet to be identified, numerous purine transporters have now been cloned. All protozoan purine transporter-encoding genes characterised to date have been of the Equilibrative Nucleoside Transporter family conserved in a great variety of eukaryote organisms. However, these protozoan transporters have been shown to be sufficiently different from mammalian transporters to mediate selective uptake of therapeutic agents. Recent studies are increasingly addressing the structure and substrate recognition mechanisms of these vital transport proteins.

Purine-less death in Plasmodium falciparum induced by immucillin-H, a transition state analogue of purine nucleoside phosphorylase

Plasmodium falciparum is responsible for the majority of life-threatening cases of malaria. Plasmodia species cannot synthesize purines de novo, whereas mammalian cells obtain purines from de novo synthesis or by purine salvage. Hypoxanthine is proposed to be the major source of purines for P. falciparum growth. It is produced from inosine phosphorolysis by purine nucleoside phosphorylase (PNP). Immucillins are powerful transition state analogue inhibitors of mammalian PNP and also inhibit P. falciparum PNP as illustrated in the accompanying article (Kicska, G. A., Tyler, P. C., Evans, G. B., Furneaux, R. H., Kim, K., and Schramm, V. L. (2002) J. Biol. Chem. 277, 3219-3225). This work tests the hypothesis that erythrocyte and P. falciparum PNP are essential elements for growth and survival of the parasite in culture. Immucillin-H reduces the incorporation of inosine but not hypoxanthine into nucleic acids of P. falciparum and kills P. falciparum cultured in human erythrocytes with an IC(50) of 35 nm. Growth inhibition by Imm-H is reversed by the addition of hypoxanthine but not inosine, demonstrating the metabolic block at PNP. The concentration of Imm-H required for inhibition of parasite growth varies as a function of culture hematocrit, reflecting stoichiometric titration of human erythrocyte PNP by the inhibitor. Human and P. falciparum PNPs demonstrate different specificity for inhibition by immucillins, with the 2'-deoxy analogues showing marked preference for the human enzyme. The IC(50) values for immucillin analogue toxicity to P. falciparum cultures indicate that inhibition of PNP in both the erythrocytes and the parasite is necessary to induce a purine-less death.

Altered membrane permeability: a new approach to malaria chemotherapy

During its development: in the host erythrocyte, the malarial parasite causes profound alterations in the permeability of the host cell membrane. Nucleoside transport pathways, which are induced by the parasite in the host erythrocyte membrane, have properties significantly different from those of the host cell. Here, Annette Gero and Joanne Upston review the current knowledge o f the parasite-induced transporters and show that they can be used to selectively direct cytotoxic compounds into the parasite-infected cell, thereby indicating their chemotherapeutic potential.

Plasmodium falciparum: ATP/ADP transport across the parasitophorous vacuolar and plasma membranes

Previous studies have shown that ATP is required for the growth of the intracellular parasite, Plasmodium, outside its host cell, the erythrocyte, and that bongkrekic acid, an inhibitor of mitochondrial ATP/ADP transporter, inhibits intraerythrocytic Plasmodium maturation. We have characterized ATP/ADP transport of Plasmodium falciparum, isolated by either immune lysis or N2-cavitation. [3H]ATP uptake was due to ATP/ADP exchange since ADP efflux was dependent on exogenous ATP in an approximate 1:1 stoichiometry and both ATP influx and ADP efflux were equally inhibited by atractyloside (Ki = 100 nM). ATP uptake was not inhibited by the nucleoside transport inhibitor, nitrobenzylthioinosine. Conversely, adenosine and hypoxanthine transport were insensitive to atractyloside. ATP influx was characterized by a Km = 0.14 mM and Vmax = 1.2 nmol ATP/min/10(6) cells. Substrate specificity studies for nucleotide-induced ADP efflux indicated a preference for an adenosine ring and triphosphate, but transport did not require a hydrolyzable phosphate bond. Protein synthesis was measured with free parasites starved of glucose. Addition of 1.0 mM ATP resulted in a 40% recovery of total protein synthetic capacity in a process inhibited by 500 nM atractyloside, suggesting that uptake of erythrocyte-derived ATP by P. falciparum may be essential for maintaining maximal rates of protein synthesis during specific stages of intra-erythrocytic parasite maturation.

The Wellcome Trust lecture. Mechanisms of molecular trafficking in malaria

The asexual stages of Plasmodium living within the erythrocyte result in growth-related changes in the permeability properties of the red cell for substances such as glucose, amino acids, purine nucleosides, sodium, potassium, calcium, zinc, iron and several antimalarial drugs such as chloroquine, amodiaquine and mefloquine. In most cases such changes do not appear to be due to a modification in the affinity or specificity of red cell transporters; indeed, for most substances the membrane-associated transporters are either unaffected or are partially inactivated. In malaria-infected erythrocytes, where a striking increase in influx has been observed, it has been attributed to the insertion of parasite-encoded transporters into the red cell membrane or the formation of aqueous leaks and/or pores. Leak formation, in the vast majority of cases, does not appear to be dependent on the insertion of plasmodial proteins into the red cell membrane. However, since the data presently available are less than satisfactory for discriminating amongst the various possible transport mechanisms future studies will require painstaking efforts and carefully controlled conditions to discriminate amongst the various transport systems which are operational in the malaria-infected red cell and the parasite.

Efflux of 3H-thymidine by erythrocytes from mice infected with Trypanosoma brucei brucei

Erythrocytes from mice infected with Trypanosoma brucei brucei showed a higher rate of efflux of labelled thymidine than did control erythrocytes from uninfected mice (0.56 +/- 0.10 and 0.38 +/- 0.06 mumole min-1 ml-1 packed cells respectively). Efflux of the nucleoside from erythrocytes of normal and infected mice were inhibited to the same extent by a specific nucleoside transport inhibitor, nitrobenzylthioinosine. Enumeration of nitrobenzylthioinosine binding sites on the erythrocytes showed that both have similar numbers of sites (6.2-6.6 X 10(3) sites/erythrocyte). It is concluded that the membrane permeability of the erythrocytes from infected mice was affected by the trypanosome in such a way as to enhance the purine nucleoside transport capacity. This may result in an increased supply of vital purine bases and nucleosides to trypanosomes which depend on their hosts for these nutrients.

Extracellular development of Plasmodium knowlesi erythrocytic stages in an artificial intracellular medium

The development of erythrocytic stages of Plasmodium knowlesi separated from their host cells has been determined in terms of the capacity of the isolated organisms to carry out the synthesis and secretion of proteins. P. knowlesi trophozoites and schizonts were released from host cells by nitrogen decompression and cultivated in a medium consisting of 20 mM Na+; 120 mM K+; 1 mM Mg2+; no Ca2+; 100 mM Cl-; 20 mM HCO3-; 5 mM Hepes [pH 6.73], glucose, vitamins, amino acids and 10% fetal calf serum. The yield was about 97% intact parasites, judging by their ability to maintain a membrane potential, and these parasites had more than 80% the capacity of infected cells for nuclear replication and macromolecule biosynthesis. Pulse and pulse-chase labeling studies with [35S]methionine show that parasite-synthesized proteins with Mr 160 000, 140 000, 100 000 and 58 000 are exported from the parasite in soluble form. Proteins with Mr 140 000, 100 000, 58 000-60 000, 40 000 were recovered in a particulate fraction isolated from the parasite culture fluid. An Mr 62 000 protein synthesized in large amounts by isolated parasites during the last 2h of the developmental cycle, could not be detected in infected erythrocytes, and a minor early Mr 74 000 protein becomes prominent in free parasites but not infected cells toward the end of the developmental cycle. Parasite-synthesized proteins with Mr 230 000, 160 000, 140 000, 62 000, 58 000 and 45 000 were labeled by incubation with radioactive N-acetylglucosamine during short term incubation in vitro. About 80% of label incorporation occurred via N-glycosylation supported by dolichol derived from the blood, and about 20% via glycolytic intermediates.

Purine metabolism by the avian malarial parasite Plasmodium lophurae

Extracts of normal duckling erythrocytes catabolized AMP to IMP, inosine and hypoxanthine; adenosine and adenine were not formed from AMP. When erythrocyte-free Plasmodium lophurae, prepared by antibody lysis, were incubated in the presence of [14C]hypoxanthine approximately 60% of the label was recovered as purine nucleotides and there was not evidence of extracellular alteration of added hypoxanthine. However, when adenosine was added to suspensions of antibody- or saponin-prepared parasites extensive conversion to inosine and hypoxanthine occurred. This conversion was found to be the result of parasite lysis with release of cytosolic purine salvage pathway enzymes; plasmodial surface membrane ecto-enzymes were not responsible for adenosine catabolism. It appears that in vivo the intracellular plasmodium utilizes the normal erythrocytic process of purine turnover to avail itself of hypoxanthine, the red cell's end product, and at the same time the parasite avoids direct competition for adenosine essential to erythrocyte survival. Since the blood plasma of infected ducklings contained increased amounts of hypoxanthine it is possible that P. lophurae also utilizes this as a purine source.

Purine metabolizing enzymes of Plasmodium lophurae and its host cell, the duckling (Anas domesticus) erythrocyte

Adenosine kinase, adenosine deaminase, hypoxanthine phosphoribosyltransferase, inosine-nucleoside phosphorylase, 5'-AMP deaminase and 5'-IMP nucleotidase were identified in cell-free extracts of duckling erythrocytes; no evidence for 5'-AMP nucleotidase and xanthine oxidase activity was found. The Km values for the duckling red cell enzymes were similar to those reported for human erythrocytes. Plasmodium lophurae extracts demonstrated similar enzyme activities except for 5'-AMP deaminase and 5'-IMP nucleotidase which were absent. It is proposed that during infection erythrocytic AMP is catabolized to IMP, inosine and hypoxanthine; the hypoxanthine is taken up by the plasmodium, utilized to form IMP, and this in turn is converted into adenine and guanine nucleotides.

Surface properties of extracellular malaria parasites: morphological and cytochemical study

Morphological and cytochemical surface characteristics of isolated malaria parasites (Plasmodium berghei) and host erythrocytes were compared by electron microscopy by using thin section and carbon replica techniques. Erythrocytes were uniform in shape and had fine, granular surfaces. In contrast, free parasites exhibited a variety of sizes, shapes, and surface textures. Fine surface stippling was a common topographical feature of isolated parasites. Small, infective forms often had patterned surfaces resulting from the protuberance of an underlying thick intermediate layer. Results of cytochemical analysis using a sialophilic colloidal iron stain indicated that the malaria parasite's surface lacked exposed sialic acid groups which would normally give rise to a net negative surface charge common to erythrocytes. Biochemical assay demonstrated that malaria parasites contained about one-half the amount of sialic acid per unit weight as did control red cell extracts. Similarly, external acidic mucopolysaccharide coats of free parasites, as revealed by ruthenium red staining were extremely thin as compared with the thick glycocalyx layer of red cells. Lipid plaques at the surface of parasites and red cells were localized by lipophilic iron colloid staining. Although the gross patchwork distribution of plaques was somewhat similar for the two cell types, the parasites were stained more intensely and had a closer-knit patchwork pattern than those exhibited by the erythrocytes. Such findings indicate that there are slight differences in the arrangement of phospholipids at the surfaces of limiting membranes of host cells and parasites. The significance of the above cytochemical surface properties of the malaria parasite (which are seemingly akin to those of intracellular organelles is discussed in relation to certain host-parasite interactions, such as parasite adhesion to target cells and enhanced clearance of extracellular parasites.